The invention is concerned with an improved and relatively inexpensive system for use on an aircraft, or the like, and which employs a combination of a doppler radar system and an inertial system to provide navigational and fire control information for the aircraft. Navigational systems employing combinations of inertial elements and doppler radar units are known in the art. Such systems are described, for example, in U.S. Pat. Nos. 2,914,763 -- Greenwood, 3,028,592 -- Parr, 3,430,238 -- Buell et al. and 3,430,239 -- Buell et al.
U.S. Patent 3,430,238, for example, describes a high quality inertial heading reference unit mounted on a doppler antenna to measure vertical error of the antenna stabilizer. The derived error signal is then used in the system to impart a compensation to the vertical gyroscope which supplies pitch and roll data to the doppler antenna. It is to be noted that any air frame flexure or bending between the vertical gyroscope and the antenna produces an uncompensated error in the system. The system described in U.S. Pat. No. 3,430,239 attempts to eliminate the effects of air frame flexure by using a two gimbal inertial quality vertical gyroscope in place of the vertical gyroscope described in the system of U.S. Pat. No. 3,430,238. In the system U.S. Pat. 3,430,239, doppler radar velocity signals are used to stabilize the vertical gyroscope to compensate for vehicle motion over the earth. The vertical gryoscope may be mounted anywhere in the aircraft, and it still provides extremely accurate pitch and roll information at the location at which it is mounted.
The system of the present invention is also capable of providing the accurate pitch and roll information anywhere in the aircraft, but without the need for the expensive high quality vertical gyroscope used in the system described in U.S. Pat. No. 3,420,239. The system of the invention provides accurate vertical information anywhere in the aircraft, and at a cost only slightly greater than the uncompensated system described in the U.S. Pat. No. 3,430,238.
As described in the U.S. Pat. No. 3,420,239, doppler and inertial systems have complementary advantages, in that the doppler system has good response to low frequency fluctuations of input data, while inertial systems have good response to high frequency fluctuations of input data. One factor affecting the accuracy of both systems is the need for accurate knowledge of the vertical direction. This knowledge is required in doppler systems in order to determine the angular position of the transmitted beams, although recent developments in the art of linear array antennas have relaxed the requirements to some extent. The accurate knowledge of the vertical direction is a stringent requirement in the inertial system because the platform must be exactly horizontal if the accelerometers are to interpret the acceleration of gravity purely as vertical acceleration.
Doppler inertial navigation systems are usually expected to provide not only accurate heading and velocity data for navigational purposes, but also an accurate vertical reference. The expression "vertical reference", when used in the following specification, is intended to refer to the problem of accurately determining the orientation in the aircraft at a particular location which represents an exact vertical vector. Such vertical reference data, in addition to the requirements described above, is often required for the accurate operation of auxiliary equipment such as aerial cameras, terrain following systems, bomb sights, and fire control apparatus.
The embodiment of the attitude reference system to be described herein, for purposes of explanation, will be considered as providing fire control signals for a swivelling gun station in an aircraft. It has been found that if the swivelling gun station requiring the accurate vertical reference is located in a position remote from the navigational system, then the vertical data supplied by the navigation system does not accurately represent the vertical direction at the position of the apparatus. This is because of factors such as physical flexure and deformation of the air frame between the location of the navigation system and the swivelling gun station.
As mentioned above, the aforesaid problem is overcome in the system of U.S. Pat. No. 3,430,239 by the provision of a high quality vertical reference unit at the location, for example, of the swivelling gun station. The system of the present invention, however, permits the elimination of the high quality, expensive vertical reference unit, and thereby provides a major cost reduction in the overall system, while yet retaining a high degree of accuracy in the overall control.